Raman spectroscopy is a well-known technique for analyzing molecules or materials. In conventional Raman Spectroscopy, an analyte (or sample) that is to be analyzed is irradiated with high intensity monochromatic radiation provided by a radiation source, such as a laser. A radiation detector detects radiation that is scattered by the analyte. The properties of the scattered radiation provide information relating to the analyte.
FIG. 1 schematically illustrates a conventional Raman system 10. The Raman spectroscopy system 10 includes an electromagnetic radiation source 12 that is configured to emit incident electromagnetic radiation 18, an analyte stage 16 on which an analyte (not shown in FIG. 1) may be positioned, and an electromagnetic radiation detector 14. The radiation detector 14 is configured to detect at least a portion of scattered radiation 20 that is scattered by the analyte. The Raman spectroscopy system 10 also includes various optical components 30 positioned between the electromagnetic radiation source 12 and the analyte stage 16 (for example, lens 32A and filter 34A), and between the analyte stage 16 and the detector 14 (for example, lens 32B and filter 34B).
The electromagnetic radiation source 12 may be a commercially available laser. The wavelength or wavelengths of incident electromagnetic radiation 18 that may be emitted by the electromagnetic radiation source 12 typically are in the visible region to the near infrared region of the electromagnetic spectrum.
The radiation detector 14 receives and detects at least a portion of the scattered radiation 20 that is scattered by an analyte disposed on the analyte stage 16. The detector 14 may include a device for determining the wavelength of the scattered radiation 20 (for example, a monochromator) and a device for determining the intensity of the scattered radiation 20 (for example, a photomultiplier). Typically, the scattered radiation 20 is scattered in all directions relative to the analyte stage 16.
Optical components 30 positioned between the electromagnetic radiation source 12 and the analyte stage 16 are used to collimate, filter, or focus the incident radiation 18 before the incident radiation 18 impinges on the analyte stage 16. Optical components 30 positioned between the analyte stage 16 and the detector 14 are used to collimate, filter, or focus the scattered radiation 20.
Referring to FIG. 2, an analyte 26 may be provided on the analyte stage 16 of the Raman spectroscopy system 10 (shown in FIG. 1) and irradiated with the incident radiation 18 emitted by the electromagnetic radiation source 12 to perform Raman spectroscopy using the Raman spectroscopy system 10. As the incident radiation 18 impinges on the analyte 26, at least some of the incident radiation 18 will be scattered by the analyte 26. FIG. 2 illustrates scattered radiation 20 that has been scattered by the analyte 26. A majority of the photons of the incident radiation 18 that impinge on the analyte 26 are elastically scattered by the analyte 26. In other words, the scattered photons have the same energy, and thus the same wavelength, as the incident photons. This elastic scattering of photons is termed “Rayleigh scattering,” and radiation consisting of these elastically scattered photons is termed “Rayleigh scattered radiation” or “Rayleigh radiation.” FIG. 2 also illustrates Rayleigh scattered radiation.
The Rayleigh scattering process can be further described with reference to the simplified Jablonski diagram shown schematically in FIG. 3, which illustrates various energy levels of a hypothetical analyte, such as the analyte 26 shown in FIG. 2. In FIG. 3, energy levels of the analyte are represented as horizontal lines. As seen therein, the ground state energy level (the lowest energy level) is shown at the bottom of the diagram, excited vibrational energy states are shown just above the ground state, excited electronic energy states are shown at the top of the diagram, and virtual excited states are shown between the excited electronic states and the excited vibrational states. As seen in FIG. 3, Rayleigh scattering typically involves absorption of a single photon of the incident radiation 18 by the analyte 26, which causes the analyte 26 to transition from the ground state to a virtual state followed by relaxation to the ground state. As the analyte 26 relaxes to the ground state, the analyte 26 emits a photon of scattered radiation 20 that has energy equal to that of the photon of the incident radiation 18. In this manner, the photon of the incident radiation 18 is considered to have been elastically scattered.
In addition to the Rayleigh scattering of photons, a very small fraction of the photons of the incident radiation 18 may be inelastically scattered by the analyte 26. Referring again to FIG. 2, Raman scattered radiation 22 is also emitted from the analyte 26. Typically, only about 1 in 107 of the photons of the incident radiation 18 is inelastically scattered by the analyte 26. These inelastically scattered photons have a different wavelength than the photons of the incident radiation 18. This inelastic scattering of photons is termed “Raman scattering,” and radiation consisting of Raman scattered photons is termed “Raman scattered radiation” or “Raman radiation.” The photons of the Raman scattered radiation can have wavelengths less than, or more typically, greater than the wavelength of the photons of the incident radiation 18.
The Raman scattering process can be further described with reference to the simplified Jablonski diagram shown in FIG. 3. When a photon of the incident radiation 18 collides with the analyte 26, energy can be transferred from the photon to the analyte 26, or from the analyte 26 to the photon. When energy is transferred from the photon of the incident radiation 18 to the analyte 26, the Raman scattered photon will have a lower energy and a corresponding longer wavelength than the incident photon. These Raman scattered photons having lower energy than the incident photons are collectively referred to in Raman spectroscopy as the “Stokes radiation.” As seen in FIG. 3, 1st order Stokes Raman scattering typically involves absorption of a single photon of the incident radiation 18 by the analyte 26, which causes the analyte 26 to transition from a first energy state (for example, the ground state) to an excited virtual state. The analyte 26 then relaxes to an excited vibrational state of higher energy than the first energy state. As the analyte 26 relaxes to the excited vibrational state, the analyte 26 emits a photon of scattered radiation 20 that has less energy (and a longer wavelength) than the photon of the incident radiation 18. In this manner, the photon of the incident radiation 18 is considered to have been inelastically scattered.
When energy is transferred from the analyte 26 to a photon of the incident radiation 18, the Raman scattered photon will have a higher energy and a corresponding shorter wavelength than the photon of the incident radiation 18. These Raman scattered photons, which have higher energy than the incident photons, are collectively referred to in Raman spectroscopy as the “anti-Stokes radiation.” As seen in FIG. 3, 1st order anti-Stokes Raman scattering typically involves absorption of a single photon of the incident radiation 18 by the analyte 26, which causes the analyte 26 to transition from an excited vibrational energy state to an excited virtual state. The analyte 26 then relaxes to a lower energy state (for example, the ground state) than the excited vibrational energy state. As the analyte 26 relaxes to the lower energy state, the analyte 26 emits a photon of scattered radiation 20 that has more energy (and a shorter wavelength) than the photon of the incident radiation 18. In this manner, the photon of the incident radiation 18 is considered to have been inelastically scattered.
The shift in energy (wavelength, frequency, or wave number) of the Raman scattered photons in relation to the Rayleigh scattered photons is known as the “Raman shift.”
Raman scattering primarily involves one photon excitation—one photon relaxation process. These Raman scattering processes are often referred to as “1st order” Raman scattering processes. However, multiple photon excitation—single photon relaxation processes are also observed and are referred to as “hyper Raman scattering” processes. Two photon excitation—one photon relaxation scattering processes are referred to as “2nd order” hyper Raman scattering processes, three-photon excitation—one photon relaxation processes are referred to as “3rd order” Raman scattering processes, etc. These higher order Raman scattering processes are often referred to as “harmonics.”
In 2nd order scattering processes, a molecule of the analyte 26 in an initial energy state absorbs the energy from two photons of the incident radiation 18 causing an energy transition in the analyte 26 to a virtual excited state, followed by relaxation to a final energy state and emission of a single scattered photon. If the final energy state is the same as the initial energy state, the scattering process is referred to as hyper Raleigh scattering (not represented in FIG. 3). If the final energy state is higher than the initial energy state, the scattering process is referred to as 2nd order Stokes hyper Raman scattering. Finally, if the final energy state is lower than the initial energy state, the scattering process is referred to as 2nd order anti-Stokes hyper Raman scattering. The Stokes and anti-Stokes 2nd order hyper Raman scattering processes are also represented in the Jablonski diagram shown in FIG. 3.
The Raman scattered radiation that is scattered by the analyte 26 (including the hyper Raman scattered radiation) is often referred to as the “Raman signal.”
Information may be obtained from hyper Raman scattered radiation that cannot be obtained from 1st order Raman scattered radiation. In particular, vibrational information may be suppressed in Raman scattered radiation due to symmetry issues, thereby resulting in what are often referred to as “silent modes.” These silent modes may not be suppressed in the hyper Raman scattered radiation.
Referring again to FIG. 2, when the analyte 26 is irradiated with the incident radiation 18, the scattered radiation 20 may include Raman scattered radiation 22, which may comprise 1 st order Raman scattered radiation 22A (Stokes and anti-Stokes) and higher order hyper Raman scattered radiation 22B (Stokes and anti-Stokes), in addition to Rayleigh scattered radiation 24.
The Raman scattered radiation 22 is detected using the radiation detector 14. The wavelengths and corresponding intensity of the Raman scattered radiation 22 may be determined and used to provide a Raman spectral graph. Analytes 26 generate unique Raman spectral graphs. The unique Raman spectral graph obtained by performing Raman spectroscopy can be used to obtain information relating to the analyte 26 including, but not limited to, the identification of an unknown analyte 26, or the determination of physical and chemical characteristics of a known analyte 26.
Raman scattering of photons is a relatively weak process relative to Rayleigh scattering, and hyper Raman scattering is even weaker. Conventional Raman systems are designed primarily to detect 1st order Raman scattered radiation, which has wavelengths proximate the wavelength of the Rayleigh scattered radiation. The detector 14 is capable of detecting the high-intensity Rayleigh scattered radiation 24 in addition to the low-intensity Raman scattered radiation 22. The detection of the Raman scattered radiation 22 may be difficult due to the high intensity of the Rayleigh scattered radiation 24. To overcome this difficulty, the optical components 30 positioned between the analyte stage 16 and the detector 14 include a radiation filter 34B that prevents the Rayleigh scattered radiation 24 from being detected by the detector 14, thus allowing only the Raman scattered radiation 22 to be received by the detector 14. Commercially available notch filters may be used for such purposes. Notch filters are typically very expensive and add to the bulk and fragility of any detector for Raman scattered radiation. As such, eliminating the need for a filter would allow for smaller and cheaper Raman spectroscopy systems. Accordingly, there is a need for Raman spectroscopy systems that may operate without a filter between the analyte 26 and the detector 14.
Surface-enhanced Raman spectroscopy (SERS) is a technique that allows for enhancement of the intensity of the Raman scattered radiation relative to conventional Raman spectroscopy. In SERS, the analyte typically is adsorbed onto or placed adjacent to what is often referred to as a SERS-active structure. SERS-active structures typically include a metal surface or structure. Interactions between the analyte and the metal surface may cause an increase in the intensity of the Raman scattered radiation.
Several types of metallic structures have been employed in SERS techniques to enhance the intensity of Raman scattered radiation that is scattered by an analyte. Some examples of such structures include electrodes in electrolytic cells, metal colloid solutions, and metal substrates such as a roughened metal surface or metal “islands” formed on a substrate. For example, it has been shown that adsorbing analyte molecules onto or near a specially roughened metal surface of gold or silver can enhance the Raman scattering intensity by factors of between 103 and 106.
Raman spectroscopy recently has been performed employing metal nanoparticles, such as nanometer scale needles, particles, and wires, as opposed to a simple roughened metallic surface. This process will be referred to herein as nano-enhanced Raman spectroscopy (NERS). Structures comprising nanoparticles that are used to enhance the intensity of Raman scattered radiation may be referred to as NERS-active structures. The intensity of the Raman scattered radiation that is scattered by an analyte adsorbed on such a NERS-active structure can be increased by factors as high as 1016.